This invention relates to a method for the spectroscopic determination of the amount of one constituent of a fluid mixture (e.g., a liquid mixture) in another constituent of the mixture following the separation of the mixture into its constituents. More particularly though not exclusively, the invention relates to the determination of the entrained oil content of wax resulting from the separation, in a dewaxing filter, of a waxy raffinate into dewaxed hydrocarbon oil boiling in the lubricating oil range (hereinafter "dewaxed oil" or DWO) and wax. A modification of the invention relates to measuring the amount of one constituent in the fluid mixture. The amount of the one constituent in the other constituent or in the fluid mixture itself can be expressed as a molar, weight or volume percentage, fraction, or ratio, or an actual weight or volume if the corresponding weight or volume of the other constituent or feed were also known. A particular advantage of the invention is that the method is insensitive to changes in the feed composition or upstream process conditions, as will be explained in more detail hereinbelow.
Petroleum distillates obtained at atmospheric pressure and at temperatures above 370.degree. C., and from further vacuum distillation, can be converted into lubricating oil basestocks. The conversion can be achieved through solvent extraction of a portion of the aromatic species present in the oil, followed by solvent dewaxing, or through other techniques, well-known in the art. Further processing, such as deoiling of the wax, may also be carried out.
In the dewaxing process, the wax is caused to precipitate from the waxy feed at a particular temperature, solvent, and solvent-to-oil ratio, and is separated from the oil by filtration. Oil may be entrained in the wax during this process. Economic credits may be obtained by optimizing the entrained oil content of the wax.
Several standard tests can be used to measure the entrained oil content. For example two tests of the American Society of Testing Materials, ASTM D721 and D3235, are currently used to determine the oil content of wax for sale. The ASTM D721 test serves as the American industry definition of the oil content of wax for oil levels below 15 wt %. Neither test, however, gives an accurate measure of the entrained oil content, since the tests report as oil the solvent-extractable portion of the wax at the test conditions. This material is not necessarily the same as the entrained oil in the wax, in quantity or composition, since the solvent and extraction conditions used in the ASTM tests may differ significantly from those used in the plant dewaxing.
In general, under the ASTM test conditions, not all of the wax is insoluble and not all of the oil is soluble. Consequently, for lower temperature distillation fractions, comprising lower molecular weight species, the ASTM tests are likely to report more oil than is actually entrained, since some of the wax and essentially all of the oil is soluble in the test solvent. Alternatively, for higher temperature distillation fractions, comprising high molecular weight species, the ASTM tests are likely to report less oil than is actually entrained, since some of the oil and essentially all of the wax is insoluble in the test solvents. Thus, in general, the ASTM test methods do not report the correct amount of entrained oil in wax. Finally, the ASTM methods are time-consuming and require considerable operator skill in order to achieve the claimed precision, and are not amenable to on-line implementation.
Practical needs demand a method preferably for measuring the oil content of the wax, but which has general application to determining the content of a first constituent of a fluid mixture (following separation into that one constituent and at least one further constituent) either in the fluid mixture itself prior to the separation or in one or more further constituents.
Furthermore, the method should be suitable for on-line measurement or batch measurement, as circumstances require. A measurement method in which the need for dilution can be avoided and/or where rapid measurements can be made which can be used to control the separation process, is desirable.